Phase-lock loops are used particularly in receivers to generate an oscillator signal with which a received signal is converted to an intermediate frequency. This mixing process is known as an intermediate frequency conversion. An intermediate frequency conversion to a particular intermediate frequency of 0 is especially advantageous, since a signal thus converted can be further processed especially easily. An example of a possible problem in a frequency conversion can be explained based on a conversion of OFDM signals (orthogonal frequency division multiplex signals). OFDM signals are pulsed signals, and comprise several individual subcarriers of different frequencies. Each subcarrier transmits one symbol per pulse, which is given by the amplitude and the phase position of the subcarrier during the pulse. The totality of all subcarriers per pulse is called the OFDM symbol.
The individual subcarriers are distinguished by the fact that the cross correlation between any two subcarriers ideally results in a value of 0. In other words, in the frequency space, a zero crossing of a subcarrier is always within the maximum of an adjacent carrier. The frequencies of the individual subcarriers differ by n times a difference frequency.
For receivers that receive OFDM signals, there is the problem of a DC signal component in the signal path of the receivers, which is called the DC offset. This is important primarily in the case of a slight frequency offset of the received signal with respect to the ideal receiving frequency. With the conversion in the receiver, the DC signal component is converted to the center frequency normally not used by OFDM signals. The later signal processing can compensate for this, however. In case of an additional frequency offset in the received OFDM signal, all subcarriers of the OFDM signal shift by the value of the frequency offset. This can cause a subcarrier of the OFDM signal to overlap with the DC signal component. The overlap can no longer be compensated for by the subsequent signal processing and is expressed in an erroneous demodulation of the subcarrier overlapped by the DC signal component.
The problem of a DC signal component with simultaneous frequency offset of the carrier can be easily corrected with OFDM receivers that convert to an intermediate frequency that is not equal to 0. On the other hand a frequency conversion to the intermediate frequency 0 is desirable, particularly for send mode and for various modulation types, since such building elements can be used for both sending and receiving mode.
OFDM transceivers in which the sending and receiving unit are implemented together thus use switching for the intermediate frequency generation to solve the above problem of a shifted DC signal component. The sending operation and the receiving operation thus use different local oscillator signals so that in the receiving operation the intermediate frequency is no longer 0 but at least half the signal bandwidth. A DC signal component in the receiver path then no longer falls within the useful signal band.
Since such switching must take place within microseconds, it is hardly possible to switch a normal phase-lock loop as a local oscillator using corresponding control. Rather, an additional mixer is used, which converts the local oscillator signal with an auxiliary signal of a suitable frequency and thus generates the desired frequency-shifted signal. A mixer generates interference signals, however, such as harmonics of the intermediate frequency or image signals. Due to this, the problem of undesirable signal portions on the intermediate frequency remains.
Another solution to compensate for a DC signal component in the case of a frequency offset of an OFDM signal meeting WLAN standard 802.11a can be found in a publication by A. Behzad et. al. Entitled “20.4 Direct conversion CMOS Transceiver with Automatic Frequency control for 802.11a Wireless LANs”, ISSCC 2003, Session 20, Paper 20.4. The method described there is very cumbersome, however.